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Research

I work in theoretical astrophysics, specifically galaxy clusters. I prefer doing analytical, semi-analytical and simulation-based works.
I am also interested in other high energy astrophysical phenomena and cosmology.

Most recent results: Local thermal instability in the CGM like atmospheres

The circumgalactic medium in the lower mass halos is expected to have larger cores with constant background entropy. Such environments do not have internal gravity waves and remain disturbed over a larger radial extent. As a result, both cold gas and hot bubbles may be seen out to 50-100 kpc. We show that gas cools out along narrow filaments (K0) spreading over a relatively larger radii compared to environments with smaller cores (K0K100)  or no cores (K100). This may have interesting implications on the observations of ubiquitous cold gas (COS-Halos) along all lines of sight. 





Baryons in accreting dark matter halos: I am working on a project aiming at studying the cooling/heating and infall of gas in a dark matter halo that is accreting mass slowly from the surroundings and growing in size, over a reasonably large range of redshifts, based on numerical 1D hydro-modeling of Lagrangian shells. Currently, I am extending this model to 2D. This model is an improvement over existing idealized hydrodynamic simulations of gas in isolated halos and an ideal testbed for studying cooling-feedback-infall dynamics given a halo. For an overview, here's how the density and temperature profiles of a cluster scale halo (M14) and a small halo (M11) look like at different redshifts. The 2D maps are for the gas density of a current group scale halo at different times.






AGN Jets: I am working on the two different numerical implementations of AGN jets and the conditions under which they mix effectively with the surrounding medium in the ICM. These are very high-resolution simulations carried out in the HPC system at IISc (CRAY xc40).





Intracluster medium: For some time, I have been working on Local Thermal Instability(LTI) in the Intracluster medium(ICM). Clusters are the largest gravitationally bound systems in the universe, composed of hot X-ray emitting gas, thousands of galaxies and presumably large amounts of dark matter. Among these, cool-core clusters are known to have very short cooling times in the central regions and are believed to be stabilized by feedback heating from AGN jets emitted at the center.

The Virgo Cluster(nearest to us). The central elliptical galaxy M87 is
one of the largest in the cluster and is notable for the jet of energetic
plasma it emits. Picture courtesy: NASA
                                                                         
The system, I considered, is a cool-core cluster 
sitting in a dark matter halo, in exact thermal and hydrostatic balance. I studied the growth of small perturbations over a timescale of a few Gigayears, due to radiative cooling at very small length scales(locally), in a globally thermally stable atmosphere. I carried out a detailed perturbation analysis in the linear regime and a few simulations to study the nonlinear regime of growth, starting from small scale perturbations. I was mainly looking into the effects of geometry and gravity on thermal instability.

Here is the first and one of the higher unstable modes in the linear regime. It is worth noting that the unstable modes are all concentrated in the region of maximum BruntVäisälä oscillations.




Here is the simulation snapshot of cold gas in spherical and plane parallel atmospheres, in a "Nevarro-Frenk-White" potential well, at around 1 Gyr(in the left panel) and at later times(in the right panel), when there is considerable cold gas formation in either set-ups.

       


Here is the simulation snapshot of how cold gas is formed in an idealized potential well, with inclusion and exclusion of mid-plane to cool, in the plane-parallel atmosphere. If one includes the mid-plane for cooling, the amount of cold gas could be hugely overestimated. The spherical set-up is the most realistic one, which correctly estimates the cold gas formation.




 
Accretion: I have worked in accretion for a while during my Master's coursework at IISc. The central problem in accretion disks is how they accrete.  I started looking into the non-axisymmetric modes  in a disk and if that can sustain accretion for a long time. It turned out that the unstable disk readjusts to reach a stable state in a small time. I had also set up a disk with MRI instability and incorporate higher magnetic field effects in such a disk, to observe the effects of high plasma β in accretion.  It showed accretion within a vertically thin region near the center.